The invention relates to a method of making a premium coke and particularly to making a premium coke suitable for the commercial production of electrodes used in the melting of steel.
It is well known to use low sulfur-decant oil resulting from the catalytic cracking of petroleum to produce premium coke using delayed coking. It is known in the art that in order to produce high quality electrodes suitable for the production of steel, the coke should be so-called "needle coke" and should not contain very much sulfur. Sometimes, the needle coke is referred to as "premium coke" or "premium needle coke".
Needle coke results in an electrode possessing a low coefficient of thermal expansion (CTE) which is necessary to use the electrodes at the elevated temperatures present during the production of steel.
The low sulfur content of the premium coke is important because the commercial processes for graphitizing the electrodes include a rapid increase in temperature. Sulfur in the coke is vaporized during graphitization and the resulting internal pressure can damage the electrode. The phenomenom is commonly called "puffing".
It is a common practice to use an inhibitor to reduce puffing in the coke. There are, however, limitations on the effectiveness of the inhibitors for coke having a sulfur content of more than a given level, as determined by the rate of temperature increase and other factors.
The inhibitors are inorganic compounds and the amount of the inhibitor is limited due to the deleterious effects on the properties of the electrodes.
U.S. Pat. No. 3,624,231 to Juel et al discusses puffing in coke and the use of inhibitors.
Decant oils having less than about 1% by weight sulfur are used in prior art methods for making low sulfur premium coke using delayed coking.
It has now been determined that the critical criterion is that the premium coke should have less than about 0.8% by weight sulfur to be suitable for the commercial production of electrodes.
Because of strong demands in other commercial markets, low sulfur decant oils are becoming unavailable for the economical production of premium coke.
The invention is directed to decant oils having sulfur contents of more than about 1% by weight and generally at least about 1.5% by weight. These decant oils can contain as much as about 4% by weight although decant oils having the higher amounts are not desirable.
The instant applicants have discovered that because the sulfur content in the decant oil is distributed throughout the various molecules from the low weight molecules to the heavy weight molecules, the prior art desulfurization of decant oil is ineffective for producing low sulfur premium coke. The applicants found that the primary action of the prior art of reducing the sulfur content of a decant oil is the removal of sulfur from the low weight molecules and the coke produced by the delayed coking is primarily derived from the heaviest molecules. That is, the low weight molecules are volatilized off. As a result of this, a decant oil which has been treated according to the prior art will produce a coke having a higher sulfur content by weight than the instant hydrodesulfurized decant oil.
The applicants have realized that the hydrodesulfurization should be directed to the heavy molecules because the low molecules containing sulfur are vaporized off with their respective sulfur during the delayed coking.